1. Field
The disclosure relates generally to a linear voltage regulator circuits and, more particularly, to a linear voltage regulator circuit device having improved voltage regulation thereof.
2. Description of the Related Art
Linear voltage regulators are a type of voltage regulators used in conjunction with semiconductor devices, integrated circuit (IC), battery chargers, and other applications. Linear voltage regulators can be used in digital, analog, and power applications to deliver a regulated supply voltage.
An example of a prior art, a linear voltage regulators are illustrated in FIG. 1A. A first linear voltage regulator 10 is shown utilizing an n-type transistor pass element 40. A linear voltage regulator 10 consists of an amplifier 20, a current source 30, a pass gate 40, and a load 50 depicted by a resistor element 55 and capacitor element 60, though the load on a voltage regulator typically also includes active and inductive components. A feedback loop exists between the output of the pass gate 40 and amplifier 20. For a MOSFET-based implementation, the n-type pass transistor 40 can be typically an n-channel MOSFET device. The pass transistor 40 has a MOSFET drain connected to power supply voltage VDD, and whose MOSFET source connected to output voltage, VOUT, and whose MOSFET gate is connected to the output of amplifier 20. The amplifier 20 has a positive input defined as voltage reference input, VREF, and a negative input signal feedback voltage from the feedback loop. As illustrated in FIG. 1B, a second linear voltage regulator 110 is shown utilizing a p-type transistor pass element 140. A linear voltage regulator 110 consists of an amplifier 120, a current source 130, a pass gate 140, a load 150 depicted as a resistor element 155 and capacitor element 160, though the load on a voltage regulator typically also includes active and inductive components. A feedback loop exists between the output of the pass gate 140 and amplifier 120. For a MOSFET-based implementation, the p-type pass transistor 140 can be a typically a p-channel MOSFET device. The pass transistor 140 has a MOSFET source connected to voltage VDD, and whose MOSFET drain is connected to output voltage, VOUT, and whose MOSFET gate is connected to the output of amplifier 120. The amplifier 120 has a negative input defined as voltage reference input, VREF, and a positive input signal feedback voltage from the feedback loop.
Due to high switching currents from Class D audio amplifiers as well as the printed circuit board (PCB) impedance, the ground connection is very noisy with high voltage spikes. These voltage spikes are creating non-linear slew-rate limited perturbations on the output of the feedback amplifier. These voltage perturbations on the output of the feedback amplifier are transmitted as regulated voltage. A solution to make the design more robust to noise is to utilize a one stage operational transconductance amplifier (OTA)—as opposed to a multi-stage amplifier—as illustrated in FIG. 2. An operational transconductance amplifier 210 can consist of an amplifier with p-channel transistor loads 220A and 220B, and differential pair n-type transistor inputs 221A and 221B, a current source 230, a pass gate 240, feedback resistor divider network 250 and 251, a resistor element 252 and capacitor element 260.
A disadvantage of the single stage OTA is its low gain, limited by CMOS technology. CMOS technology has a low transconductance. A low transconductance leads to an undesirable low power supply rejection ratio (PSRR). Additionally, this also leads to a large static load dependent voltage offset, ΔVin. The voltage offset ΔVin can be defined as the current load differential (e.g. output current load ILOAD minus the typical current load ILOAD(O)) divided by the gain parameter, G.ΔVin=(ILOAD−ILOAD(O))/G As the current load, ILOAD, departs from the typical current load, ILOAD(O), a difference between the feedback voltage, VFB, and the reference voltage, VREF, is required to adjust the output current load to ILOAD Smaller is the gain, G larger will be the static load dependent voltage offset, ΔVin at the equilibrium point.
In linear voltage regulators, usage of operational transconductance amplifier (OTA) for has been discussed. As discussed in published U.S. Pat. No. 7,166,991 to Eberlein describes adaptive biasing concepts for current mode voltage regulations. Eberlein describes circuits and methods to achieve dynamic biasing for the complete loop transfer function of a current mode voltage regulator. The patent contains a pass transistor device, an operational transconductance amplifier (OTA), a feedback loop, and a feed-forward loop.
In low dropout regulators, tracking voltage divider networks have been discussed. As discussed in U.S. Pat. No. 6,703,813 to Vladislav et al., discloses a pass device, an error amplifier, a cascode device, and a tracking voltage divider. The tracking voltage divider adjusts the biasing to the cascode device.
In low dropout regulators, frequency compensation networks have been integrated into the feedback loop. As discussed in U.S. Pat. No. 6,518,737 to Stanescu et al, describes a pass transistor device, cascaded operational transconductance amplifiers (OTA), a feedback loop, a resistor divider feedback network, a frequency compensating capacitor integrated into the feedback loop.
In low dropout voltage regulators, transient boost circuits have been shown to address transient issues. Ads discussed in U.S. Pat. No. 6,046,577 to Rincon-Mora et al., describes a pass transistor device, a localized feedback loop, a resistor divider feedback network, a current mirror, and a transient boost circuit.
In these prior art embodiments, the solution to improve the response of the low dropout (LDO) regulator utilized various alternative solutions.
It is desirable to provide a solution to address the disadvantages of the operational transconductance amplifier (OTA) of large d.c. offset, low gain, and low PSRR.